CELLS. SILICA. 
59 
day — as one generation died off, it was replaced by 
another; and so, in process of time, was a stratum 
formed. 
In Bohemia there is an infusorial earth termed Kiesel- 
guhr , which is remarkable for containing peculiar shield- 
shaped discs, termed by Ehrenberg, Campy lodiscus 
clypeus ; one of these is represented by D, in Fig. 43. 
On the shores of South America, and in many islands 
of the Pacific, sea-birds, principally penguins, resort and 
deposit their excrement, which, in the lapse of ages, has 
accumulated so as to form masses of enormous extent 
and depth ; and thousands of tons have been brought to 
this country as a manure, under the name of guano. 
Guano consists of a mixture of flesh, bones, and sand, 
together with various salts of potash, lime and ammonia. 
When examined microscopically, a great abundance of 
beautiful siliceous skeletons of Diatomacece are found 
amongst it ; and curiously enough, the best samples of 
guano contain the greatest number of these remains, 
which were first detected by my late brother in 1845. 
The infusorial skeletons are chiefly circular, and many of 
them of a blue colour; they are the siliceous shells of 
animalcules once inhabiting the depths of the ocean, 
and which had been swallowed by the fishes, on which 
the penguins feed, but had not been digested; they 
belong principally to three genera: Actinocyclus , Gal- 
lionella and Coscinodiscus , and are no doubt of the 
same species with others still living in the sea in the 
neighbourhood of the Guano Islands. Now, when we 
